Membrane separation of carbon dioxide

ABSTRACT

A method for the separation of carbon dioxide from a gas mixture is described in which a dendrimer selective for carbon dioxide is present in an immobilized liquid membrane, the dendrimer being either in pure form or optionally with at least one solvent, such as but not limited to glycerol, polyethylene glycol, water, refrigerated methanol, NMP, or glycerol carbonate, the latter also having selective carbon dioxide properties as will be described below. In another embodiment, a dendrimer selective for carbon dioxide and capable of forming a film may be used in the method as the membrane itself, optionally with at least one solvent.

RELATED PATENT APPLICATION

[0001] This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 60/306,748 filed Jul. 20, 2001, whichapplication is herein incorporated by reference.

FIELD OF THE INVENTION

[0002] This invention relates to the separation of carbon dioxide fromgas mixtures. More specifically, the invention relates to a method forthe separation of carbon dioxide from a gas mixture using an ImmobilizedLiquid Membrane that contains a dendrimer selective for carbon dioxide.

BACKGROUND OF THE INVENTION

[0003] Gas separation using facilitated transport membranes (FTMs) hasbeen the subject of considerable research for many years. Majoradvantages of FTMs over conventional polymeric membranes include higherfluxes for reacting gas species like carbon dioxide, olefins and theresultant high selectivities over nonreacting species like nitrogen,paraffins etc. This is possible due to the additional mechanism of areversible chemical reaction of the preferred gaseous species with areactive carrier present in the FTM in addition to thesolution-diffusion mechanism. FTMs are particularly attractive at lowreacting species concentrations where the concentration driving forcefor the solution-diffusion membranes is very low (Meldon, J. H.;Stroeve, P.; Gregoire C. E. Facilitated Transport of Carbon Dioxide: AReview. Chem. Eng. Commun. 1982, 16, 263-300; Ho, W. S.; Dalrymple, D.C. Facilitated Transport of Olefins in Ag+-containing Polymer Membranes.J. Membr. Sci. 1994, 91, 13-25). Facilitated transport membranes includeion-exchange membranes, fixed-site carrier membranes, contained liquidmembranes, and immobilized liquid membranes (Way, J. D.; Noble R. D.Facilitated Transport. In Membrane Handbook; Ho W. S. W.; Sirkar K. K.(Eds.) Chapman and Hall, New York, 1992).

[0004] Immobilized liquid membranes (ILMs) contain a liquid solutionimmobilized in the pores of the polymeric or ceramic substrate byphysical forces. They are also referred to as supported liquid membranes(SLMs), particularly in the context when feed and sweep side are liquidstreams. The liquid solution consists of a carrier and a solvent. Thecarrier reacts reversibly with the gas species of interest.

[0005] ILMs can potentially provide the highest fluxes and selectivitiesfor reacting species such as carbon dioxide and olefins particularly atlow concentrations in gas separation. Despite the obvious advantagesoffered by the immobilized liquid membranes, commercialization of thesemembranes has not taken place due to the inherent limitation ofstability of the liquid membranes. The main reasons for the instabilityof the ILMs are due to absence of any chemical bonding of the carrier tothe substrate matrix; evaporation of the carrier species and/or thesolvent liquid into the gas phases during the operation; and lowerbreakthrough pressures associated with the liquids.

[0006] There are variations to using these liquids for CO₂ separation.The CO₂ absorption can be performed in one membrane module (withCO₂-containing gas flowing on one side, the absorbing liquid flowing onthe other side of the membrane), while the absorbing liquid isregenerated in a separate unit called stripping unit (it can be amembrane-based or a non-membrane based unit). The absorbing liquid cancontain a facilitating agent or it can contain liquids havingpreferential solubility for CO₂ over other gases, functioning as aphysical solvent. This configuration is usually calledabsorption-stripping.

[0007] Another variation is to incorporate the facilitating agent in apolymeric network and forming a thin membrane on top of a substrate. Thefacilitating agent can be incorporated into the polymer network as acomponent of the polymer solution prior to its crosslinking (Ho andDalrymple,1994, op. cit.; Ho, W. S. W. Membranes may be comprised ofsalts of amino acids incorporated into hydrophilic polymers. U.S. Pat.No. 5,611,843, Mar. 18, 1997). The facilitating agent can beincorporated into the network after forming the polymer network.(Matsuyama, H.; Teramoto, M. Facilitated Transport of Carbon Dioxidethrough Functional Membranes Prepared by Plasma Graft Polymerizationusing Amines as Carrier, in Chemical Separations with Liquid Membranes.Bartsch R. A.; Way J. D. (Eds.) ACS Symp. Series No. 642, p. 252 (1996).

[0008] The stability of aqueous-based ILMs is usually improved when thefeed and sweep sides are completely humidified, minimizing the loss ofsolvent (water) due to evaporation (Teramoto, M.; Matsuyama, H.;Yamashiro, T.; Katayama, Y. Separation of Ethylene from Ethane bySupported Liquid Membranes Containing Silver Nitrate as a Carrier. J.Chem. Eng. Japan 1986, 19, 419-424). The long term stability of thesemembranes has not been established in literature and these membranes cannot withstand even temporary oscillations in the humidity conditions oneither side of the liquid membranes. A major factor that limited thepractical applicability of such an approach is that the sweep sidealways requires a sweep gas, essentially diluting the permeated gases.This limitation has serious implications in downstream processing of thepermeate stream or when highest possible concentrations on the permeateside are required, either for economic or environmental reasons. Forexample, in the separation of carbon dioxide from gas mixtures forsequestration, the permeate side should be as concentrated astechnically possible in carbon dioxide to reduce the gas volumes forfurther transport and storage.

[0009] Another alternative way to improve the ILM stability is to uselow-volatile and hygroscopic solvents like polyethylene glycol forpreparation of the ILM (Meldon, J. H.; Paboojian, A.; Rajangam, G.Selective CO₂ Permeation in Immobilized Liquid Membranes. AIChE Symp.Set. 1986,248,114; Davis, R. A.; Sandall, O. C. CO₂/CH₄ Separation byFacilitated Transport in Amine-polyethylene Glycol Mixtures. AIChE J.1993, 39, 1135; Saha, S.; Chakma, A. Selective CO₂ Separation fromCO₂/C₂H₆ Mixtures by Immobilized Diethanolamine/PEG Membranes. J. Membr.Sci. 1995, 98, 157). However, the performance of such a membrane has notbeen acceptable (Meldon et al., 1986, op. cit.).

[0010] It is towards the use of dendrimer-containing immobilized liquidmembranes, carriers and solvents therefor in the separation of carbondioxide from gas mixtures, that the present invention is directed.

[0011] The citation of any reference herein should not be deemed as anadmission that such reference is available as prior art to the instantinvention.

SUMMARY OF THE INVENTION

[0012] In accordance with the present invention, a method for theseparation of carbon dioxide is described that uses an immobilizedliquid membrane containing a dendrimer and, optionally, at least onesolvent having carbon dioxide selectivity, such as, but not limited to,glycerol, polyethylene glycol, water, refrigerated methanol, NMP, orglycerol carbonate. Porous ceramic membranes may also be used. Othersolvents may be used. In another embodiment, the method involves using adendrimer selective for carbon dioxide and capable of forming a film asthe membrane itself, optionally with at least one solvent.

[0013] A preferred dendrimer selective for carbon dioxide is apolyamidoamine dendrimer (PAMAM), but the invention is not so limitingand other dendrimers with carbon dioxide selective properties may beused, such as those with multiple terminal amino groups and those alsowith amido groups, secondary or tertiary amines, or combinationsthereof. In a more preferred embodiment, a generation zeropolyamidoamine dendrimer is used. In most preferred embodiments, theaforementioned dendrimer is used with a glycerol solvent or withglycerol carbonate, which acts both as a solvent and as a furtherselective carbon dioxide carrier.

[0014] Any porous membrane may be used as the membrane portion of theimmobilized liquid membrane of the invention, such as but not limited toa polypropylene membrane such as CELGARD 2500 or poly(vinylidenefluoride) membranes, and preferably, hydrophilized forms of theaforementioned exemplary membranes may be used. However, the inventionis not so limiting to such membranes, and as mentioned above, apolymeric dendrimer comprising carbon dioxide selective groups, e.g.primary amino groups and secondary or tertiary amine groups, or anycombination thereof may as a film-forming material itself comprise themembrane or film, optionally with at least one solvent. Other preferredporous membranes that may be used in the invention includepolyacrylonitrile, regenerated cellulose, and polysulfone membranes.

[0015] The selection of porous membrane for use with the glycerolcarbonate carrier is as aforedescribed. Moreover, a carbon dioxideselective dendrimer or polymer with like groups may be used to form themembrane, and glycerol carbonate used as both a carrier and solvent inthe selective membrane.

[0016] The invention is directed to a method for separating carbondioxide from a feed gas comprising exposing the feed gas to anaforementioned membrane, wherein carbon dioxide is selectivelytransferred across the membrane. Thus, the feed gas is desirably reducedin its concentration of carbon dioxide. Such a desirable reduction ofcarbon dioxide in a gas is useful in a variety of applications,including but not limited to the reuse of air for animal respiration,and improving the combustibility of flammable gases that may undesirablycontain carbon dioxide. Any process where reduction or removal of carbondioxide from a gas mixture may be desired is applicable to the inventionherein.

[0017] These and other aspects of the present invention will be betterappreciated by reference to the attached drawings and DetailedDescription.

BRIEF DESCRIPTION OF THE FIGURES

[0018]FIG. 1 is an illustration of an experimental apparatus for theseparation of CO₂ from a gaseous mixture according to the invention.

[0019]FIG. 2 is a plot of the results of the experiment of Example 1

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present inventors have made improvements in the effectivenessof immobilized liquid membranes (ILMs) for the separation of carbondioxide using two approaches: selecting a carrier which itself is highlyviscous and non-volatile and does not require a solvent, and if water isneeded for its chemical reactivity, having the carrier be hygroscopic;and selecting a solvent for the carrier which is viscous andnon-volatile and may not require the presence of water for theselectivity. These approaches have been achieved by the use ofamine-containing dendrimers as carrier, and glycerol carbonate ascarrier or solvent.

[0021] Carrier refers to a compound or substance with selectivity forcarbon dioxide and can be used in a membrane for selectivelyfacilitating the transfer of carbon dioxide from one side of themembrane to the other.

[0022] As will be seen in the examples herein, generation zeropolyamidoamine (PAMAM) dendrimer in the immobilized liquid membrane(ILM) configuration was studied using flat films and hollow fibers forCO₂—N₂ separation. This dendrimer as a pure liquid functions as aCO₂-selective molecular gate with highly humidified feed gas. Thepresent work further broadens the range of relative humidity of the feedgas stream (RH_(f)) by adding a small amount of glycerol to the puredendrimer liquid. A 75% dendrimer-25% glycerol ILM was found to increasethe operating range of RH_(f) substantially while maintaining the CO₂permeance and the selectivity α_(CO2/N2) close to the levels observedwith a pure dendrimer ILM. The performances of pure and 75% dendrimerILMs were found to be superior or comparable to the highest reportedα_(CO2/N2)s. This behavior is explained in terms of the chargedenvironment in the dendrimer liquid membrane under humidified feedconditions and facilitated transport of CO₂.

[0023] The present invention focuses on these three approaches towardsimproving the stability and performance of the ILMs for gas separationfor CO₂ separation. The first approach of improving the ILM stability byemploying a liquid membrane system where a carrier is the only componentin the ILM is studied using a new class of hyperbranched polymers calleddendrimers. This class of polymers has molecular weights ranging from518 to several thousands depending on their generation. They also offerthe needed properties like non-volatility, good chemical and thermalstability, reversible complexation capability etc. (Tomalia et al.,1990).

[0024] Facilitated transport membranes provide very good selectivitiesand permeances for the reacting species (e.g. CO₂, olefins) when theyare present in low concentrations in the feed gas mixture. However,their performance at high feed side concentrations of the reacting gasesis usually compromised due to carrier saturation. The presence ofglycerol in the present ILMs does not prevent this deterioration inperformance at higher CO₂ concentrations as it is essentiallynon-selective to carbon dioxide (Chen et al., 1999). There is always aneed for better solvents which have selectivities for the gas of choice.Glycerol carbonate is suggested here as a possible CO₂-selectivephysical solvent for carbon dioxide separation.

[0025] There are several industrial processes for separating CO₂ basedon physical/chemical solvents (Kohl and Riesenfeld, 1979). In order forthe solvent based process to be practical, the solvents should havehigher solubility for CO₂ than in water, must have extremely low vaporpressure, low viscosity and low or moderate hygroscopicity. Some of thesolvents used are: methanol at low temperatures (Rectisol process),propylene carbonate (Fluor process), N-methyl-2-pyrrolidone (NMP;Purisol process), dimethyl ether of polyethylene glycol (Selexolprocess), tributylphosphate (Estasolvan process) and mixture ofdiisopropanolamine, sulfolane and water (Sulfinol process). Most ofthese solvents have higher solubility for H₂S than for CO₂. Anothercommon feature of these processes is that they are used incontactor-stripper mode, requiring two separate steps in CO₂ separation.

[0026] The following documents are provided which more fully illustratethe preferred embodiments of the invention. They should in no way beconstrued, however, as limiting the broad scope of the invention. Thesedocuments and all citations therein are incorporated herein by referencein their entireties.

[0027] 1. “Dendrimer membranes: A CO₂-selective molecular gate” by A.Sarma Kovvali, Hua Chen and Kamalesh K. Sirkar. Journal of the AmericanChemical Society 122:7594-7595, 2000.

[0028] 2. “Dendrimer liquid membranes: CO₂ separation from gas mixtures”by A. Sarma Kovvali and Kamalesh K. Sirkar. Industrial and EngineeringChemistry Research 40:2502-2511, 2001.

[0029] 3. “Dendrimer liquid membranes for CO₂ separations” a disclosureby Prof K. K. Sirkar, H. Chen and A. S. Kovvali.

[0030] 4. “Carbon dioxide separation with novel solvents as liquidmembranes” a disclosure by Prof Kamalesh K. Sirkar and A. Sarma Kovvali.

[0031] While the invention has been described and illustrated herein byreferences to the specific embodiments, various specific material,procedures and examples, it is understood that the invention is notrestricted to the particular material combinations of material, andprocedures selected for that purpose. Indeed, various modifications ofthe invention in addition to those described herein will become apparentto those skilled in the art from the foregoing description and theaccompanying figures. Such modifications are intended to fall within thescope of the appended claims.

[0032] Various publications are cited herein, the disclosures of whichare incorporated by reference in their entireties.

[0033] The present invention may be better understood by reference tothe following non-limiting examples, which are provided as exemplary ofthe invention. The following examples are presented in order to morefully illustrate the preferred embodiments of the invention. They shouldin no way be construed, however, as limiting the broad scope of theinvention.

EXAMPLE 1 Methods

[0034] The experimental setup is shown in FIG. 1. Absorber and stripperwere made of hydrophobic Celgard X-10 polypropylene hollow microporousfibers. The absorber had 40 fibers of length 17.1 cm and had agas-liquid contact area of 51.5 cm².The stripper had 80 fibers of 17.0cm length and had a gas-liquid contact area of 102 cm². A 21.66 vol % ofan aqueous solution of PAMAM (polyamidoamine) dendrimer of generationzero supplied by Dendritech (Midland, Mich.) was used as the absorbentliquid and was circulated between the absorber and stripper. Thisabsorbing liquid acts as a mobile liquid membrane. A gas mixture of 5%of CO₂ in N₂ was used as the feed gas and gaseous helium was used tostrip the CO from the absorbent liquid. The experiment was started byfirst passing the aqueous solution of dendrimer through the tube side ofthe hollow fiber membranes, followed by passing the feed and sweep gasescounter-currendy on the shell side. The feed and sweep gas flow rateswere held constant at 8.74 cc/min and 61.34 cc/min respectively. Theabsorbent liquid was also allowed to flow at a constant rate of 6.8cc/min. The experiment was continued for a period of 55 days bymaintaining constant flow rates throughout the period and frequentanalysis of the extent of CO₂ absorption.

[0035] Calculation of the Logarithmic Mean ConcentrationDifference-Based Overall Mass Transfer Coefficients of the Absorber

[0036] C_(g,in)=concentration of CO₂ in the feed inlet=5 vol%=2.0461×10⁻⁶ mol/cc

[0037] C_(g,out)=concentration of CO₂ in the feed outlet=2.55vol%=1.0461×10⁻⁶ mol/cc

[0038] V₁=liquid flow rate=6.8 cc/min

[0039] V_(g)=gas flow rate=8.74 cc/min

[0040] A_(T)=51.5 cm²

[0041] Henry's law constant=H=0.8 moles of CO₂ in liquid/moles of CO₂ ingas From the material balance, $\begin{matrix}{= \quad {{C_{l,{out}}^{t} - C_{l,{i\quad n}}^{t}} = \frac{V_{g}\left( {C_{g,{i\quad n}} - C_{g,{out}}} \right)}{V_{1}}}} & {= \quad {\left\lbrack {8.74 \times \left( {{2.046 \times 10^{- 6}} - {1.0435 \times 10^{- 6}}} \right)} \right\rbrack \div 6.8}} \\\quad & {= \quad {= {1.2886 \times 10^{- 6}{mol}\text{/}{cc}}}}\end{matrix}$

[0042] where C_(l, i  n)^(t)  and  C_(l, out)^(t)

[0043] are the total liquid phase CO₂ concentrations at the liquid inletand outlet respectively.

[0044] The quantity (ΔC)_(lin), the logarithmic mean difference of gasspecies is given by $\begin{matrix}{\left( {\Delta \quad C} \right)_{l\quad m} = \frac{\left( {{HC}_{gin} - C_{lout}^{t}} \right) - \left( {{HC}_{gout} - C_{lin}^{t}} \right)}{\ln \left( \frac{{HC}_{gin} - C_{lout}^{t}}{{HC}_{gout} - C_{lin}^{t}} \right)}} \\{= {\frac{\begin{matrix}{\left( {{0.8 \times \left( {2.0461 \times 10^{- 6}} \right)} - 0} \right) -} \\\left( {{0.8 \times \left( {1.0435 \times 10^{- 6}} \right)} - 0} \right)\end{matrix}}{\ln \left( \frac{{0.8 \times \left( {2.0461 \times 10^{- 6}} \right)} - 0}{{0.8 \times \left( {1.0435 \times 10^{- 6}} \right)} - 0} \right)} = {1.191 \times 10^{- 6}\quad {mol}\text{/}{cc}}}}\end{matrix}$

[0045] C_(l,in) and C_(l,out) and are the free CO₂ concentrations at theliquid inlet and outlet respectively and they are assumed to be zero inthe bulk of the liquid. Logarithmic mean concentration difference-basedoverall mass transfer coefficient is given by$K_{OLM} = {\frac{V_{l}\left( {C_{lout}^{t} - C_{lin}^{t}} \right)}{60{A_{T}\left( {\Delta \quad C} \right)}_{l\quad m}} = {\frac{6.8 \times \left( {1.2886 \times 10^{- 6}} \right)}{60 \times 51.5 \times 1.191 \times 10^{- 6}} = {2.38 \times 10^{- 3}\quad {cm}\text{/}\sec}}}$

Results

[0046] The results in the FIG. 2 show that the dendrimer solution isquite efficient in removing CO₂ and the absorption stripping behaviorremained unchanged over a long period of time. Therefore the pores inthe Celgard membrane was not wetted at all by the aqueous dendrimersolution, a quite advantageous effect since most aqueous solutions ofamines used for CO₂ scrubbing e.g. monoethanolamine will wet the poresof Celgard fiber and render it unusable within a few days.

What is claimed is:
 1. A method for separating carbon dioxide from a gasmixture including carbon dioxide, comprising contacting an immobilizedliquid membrane containing a dendrimer and an optional solvent saidsolvent selected from the group consisting of consisting of glycerol,polyethylene glycol, water, refrigerated methanol, NMP, glycerolcarbonate, polyethylene glycol, and combinations thereof with said gasmixture whereby at least some quantity of carbon dioxide is separated.2. The method of claim 1 wherein said immobilized liquid membranecomprises a porous hydrophilized poly(vinylidene fluoride) or apolypropylene membrane containing a dendrimer and an optional solventselected from the group consisting of consisting of glycerol,polyethylene glycol, water, refrigerated methanol, NMP, glycerolcarbonate, polyethylene glycol, and combinations thereof.
 3. The methodof claim 1 wherein the dendrimer is a generation zero polyamidoaminedendrimer.
 4. The method of claim 1 wherein the dendrimer undergoes areversible chemical reaction with carbon dioxide.
 5. A method forseparating carbon dioxide from a gas mixture including carbon dioxide,comprising in an absorbtion module, contacting a mobile liquid membranecontaining a dendrimer and an optional solvent, said solvent selectedfrom the group consisting of consisting of glycerol, polyethyleneglycol, water, refrigerated methanol, NMP, glycerol carbonate,polyethylene glycol, and combinations thereof with said gas mixturewhereby at least some quantity of carbon dioxide is separated; and in astripping unit, regenerating said dendrimer and optional solvent forsubsequent reuse in the absorbing module.